Nonlinearity of a power amplifier (PA) in a radio transmission apparatus that amplifies an RF signal used in radio communication is a factor seriously distorting the RF signal. This distortion of the RF signal causes generation of power leakage to the outside of a desired band used in transmission. Therefore, suppressing the distortion of the RF signal to be transmitted becomes an important technical issue in the transmission apparatus.
In recent years, in order to achieve higher-speed radio communication, a carrier aggregation (CA) technique for collectively using a plurality of fragmented bands disclosed in, for example, Non-Patent Document 1 has been used in some cases. In this CA technique, it is possible to secure a broadband by bringing a plurality of bands together, and to increase a transmission rate.
In addition, in an inter-band non-contiguous CA mode in which respective carrier frequencies are largely separated from each other (difference Δf between respective carrier frequencies is sufficiently larger than the modulation bandwidth fBB of an RF signal of each carrier), it is possible to improve the stability of communication by simultaneously performing communication at a plurality of carrier frequencies having different propagation characteristics.
A communication system using such a CA technique requires a signal transmission apparatus that transmits RF signals of a plurality of bands. A function of suppressing the above-mentioned distortion of the RF signal is required in such a signal transmission apparatus. In addition, from the viewpoint of a reduction in the size and cost of the apparatus, it is preferable that the transmission apparatus compatible with the CA technique can amplify and transmit RF signals of a plurality of bands using a single power amplifier.
The following Patent Documents 1 to 4 disclose a distortion compensation circuit that calculates and outputs a signal for canceling out the nonlinearity of a power amplifier that collectively amplifies RF signals of a plurality of bands, in a digital arithmetic circuit, to thereby suppress signal distortion of the RF signals which are output from the amplifier. In addition, Patent Documents 3 and 4 disclose an analog circuit having a function of compensating for in-band intermodulation distortion occurring due to frequency mixing within a single band using a look-up table or a power series. In addition, Patent Document 5 also discloses an analog circuit (radio frequency pre-distortion (RF-PD)) that calculates and outputs a signal for canceling out in-band intermodulation distortion occurring by mixing RF signals of a single band in a power amplifier. In Patent Document 5, a plurality of RF-PDs corresponding to respective frequencies are provided, and a method is disclosed in which an RF signal of a frequency corresponding to each RF-PD is selected by a switch and is input to the RF-PD, to thereby compensate for in-band intermodulation distortion occurring due to frequency mixing within a single band. The RF-PD disclosed in Patent Document 5 can be implemented using, for example, a diode linearizer disclosed in Patent Documents 6 to 8. In addition, the RF-PD disclosed in Patent Document 5 can also be implemented using an intermodulation distortion generation circuit disclosed in Patent Document 9.